Conventional magnetization produces only two poles at opposite ends of a magnet, one north and the other south. For many types of permanent magnet materials it is most commonly carried out by use of an electromagnet. The electromagnet simply comprises an iron yoke with high permeability pole pieces and coils wound about either the yoke or the pole pieces between which the material to be magnetized is positioned. A direct electric current is passed through the coils to create a magnetizing field. The magnetizing field strength varies (though non-linearly) with the amplitude of the current, if all other factors remain the same.
The magnetization of modern materials which require unusually strong magnetizing fields, such as samarium cobalt and the neodymium iron class of rare earth permanent magnets, frequently requires the use of an "impulse" type magnetizer. Impulse magnetizers are also widely used where complex pole patterns (such as band-like or "multiple" poles) are needed. A special power supply is an essential part of the impulse magnetizer; it must accomplish more than merely rectify AC to DC current. For band-like poles, special magnetizing fixtures are often used, with windings shaped like a potato masher of relatively short overall wire length. The number of turns of wire and wire sizes that can be employed in such devices are limited by the need to accommodate the current required to produce the needed magnetizing field. High currents are required for the conventional magnetization of older magnetic materials (e.g., barium ferrite) when the magnets are large and/or require a saturation field (H.sub.s) in excess of about 5000 oersteds. Extremely high currents, to produce fields up to about 45,000 oersteds, are required to saturate magnets of the rare earth type as well as to form complex multiple pole patterns, even for older materials. At present, the especially high currents needed to produce multiple poles in very narrow band-like patterns can only be developed by the sudden discharge of a large capacitor into the turns of a properly designed coil. The impulse magnetizer comprising the power supply and the fixture containing the magnetizing coil into which the current is suddenly discharged, creates a strong but transient field which lasts only for a period of a few milliseconds.
It is frequently desirable to magnetize barium, strontium and/or lead ferrite strip and sheet-like materials so that they have multiple poles, that is, poles which are in the form of parallel, alternating N-S bands on one or both faces of the material. Where holding force is the primary objective, such poles should touch at their boundaries, and the thinner the sheet or strip being magnetized the narrower the poles should be. The more fully these conditions are met, and the more fully the material is magnetized, the more strongly the resulting magnet will hold an object which is facially engaged with it. (However, the "reach" or trajectory of the flux lines in the region around the strip diminishes with narrowing of the poles.) As a practical matter, where band-like poles are to be formed with an impulse magnetizer, the area and depth to which a strip or sheet can be magnetized is restricted by the current required (and thus by the size and total length of wire which may be used), as well as by the total number of poles to be formed concurrently per unit width of the strip by each discharge of the magnetizer. For example, if an exceptionally good impulse magnetizer is used to form about 18 poles per inch (of width) on 0.030" thick commercial barium ferrite composite, only a volume of about 0.25 to 0.75 cubic inches of material can be effectively magnetized by each discharge of the impulse magnetizer. Because of the limited volume of material which can be magnetized with a single discharge, in order to magnetize a long strip the capacitor must be recharged after each discharge, the strip indexed or advanced, the capacitor again discharged, the strip again indexed, and so on repetitively. This, of course, substantially slows the process of multiple pole magnetization. Moreover, impulse magnetizers are noisy (the discharge creates a sudden crack or report); further, they overheat, fail dielectrically, break down, represent potential electrical hazards, and are quite expensive to build or purchase.